We propose to study the hypothesis that fluid exchange and balance in muscle is strongly dependent on the pattern of vasomotor activity of arterioles. In our model flow and exchange are regulated by vasomotor activity, in such a fashion that when the arterioles dilate, the capillary bed filters fluid, and when it constricts, capillary pressure equilibrates with venular pressure and the capillaries absorb interstitial fluid. Studies will be carried out in the microvasculature of the rat spinotrapezius, the rabbit tenuissimus muscle and the nailfold. Vasomotor activity of arterioles will be characterized in terms of the sequence in which this phenomenon is propagated through the arteriolar tree, the displacement vs. time trajectory of the arteriolar wall and length of the active portion of the vessel. Flow and pressure measurements will be made in the capillaries that originate from active arterioles. The effect of the motion of the arteriolar wall on the flux of tissue fluid and tissue fluid pressure will be analyzed theoretically. The changes in hematocrit that occur in conjunction with flow changes due to vasomotor activity will be studied to determine how they affect tissue perfusion. Fluorescein labeled albumin and dextran will be utilized to enhance visibility and gain visual access to vessels below the surface.